Petroleum is found in subterranean formations or reservoirs in which it has accumulated. In many cases the petroleum may be recovered by penetrating the reservoir with a well and allowing the fluid to flow to the surface as a result of natural pressure existing in the reservoir or, where there is insufficient natural pressure, by pumping the fluid to the surface. However, in many reservoirs the petroleum is too viscous to flow to the well by natural forces or to be economically pumped to the well. This type of petroleum is commonly known as "heavy oil'8. An example of a formation which contains an extremely viscous type of heavy oil is the Athabasca tar sands formation located in Alberta, Canada. The heavy oil in this formation, and in various other formations located throughout the world, is commonly known as "bitumen".
The processes used to recover bitumen from tar sand deposits include surface mining and in-situ processes. Generally, with surface mining processes, the overburden of the formation is mechanically removed and the tar sand is mechanically broken-up and transported to the processing site so the bitumen can be removed from the sand. However, surface mining is usually used only when the overburden of the formation is thin enough that it can be economically removed. When surface mining is not economically feasible, in-situ processes are often used instead.
In-situ processes utilize techniques for producing the bitumen from the sand within the tar sands deposit, rather than on the surface. Accordingly, the bitumen is transported to the surface and a major portion of the sand is left in the tar sands deposit. Some of these processes, commonly known as thermal recovery methods, include cyclic steam simulation, steam flooding, and in-situ combustion. Generally, thermal recovery methods utilize steam to input mass and heat energy to the reservoir in order to reduce the viscosity of the bitumen and supplement the drive energy of the reservoir, thereby enabling the bitumen to flow through the reservoir to the well at economic rates.
Although thermal recovery methods have proven to be viable processes, they require large capital investments for steam generation, steam transmission, and hot fluid treating facilities. The operating costs associated with these methods are usually high because of the cost of fuels required to generate steam. In addition, the success of thermal recovery methods is very dependent on reservoir quality. Relatively small variations in bitumen and water saturations, reservoir thickness, and clay content, among other reservoir characteristics, may have a significant impact on production rates and ultimate bitumen recovery.
Because of the problems associated with surface mining and in-situ separation, various hydraulic mining methods have been suggested as alternatives for recovery of bitumen from tar sands. Historically, hydraulic mining methods have been used to recover gold, coal, phosphate, uranium, and bauxite ores. One hydraulic mining method proposed for recovery of bitumen from tar sands is commonly known as "borehole mining". Generally, borehole mining is a process whereby the reservoir matrix and the fluid it contains are physically removed from the reservoir by the cutting and erosive action of water jets which access the reservoir from wellbores. The reservoir matrix and associated fluid are produced as a slurry by circulation and are transported by slurry pipeline to a plant for processing. Following completion of the mining operations, the reservoir cavity is backfilled with produced sand to control subsidence and to minimize the volume of sand which must be stored on the surface or otherwise disposed of.
Although various borehole mining methods have been proposed for recovering bitumen from tar sands, none utilize the novel enhancements described herein.